At a terminal connection part (terminal) or an intermediate connection part (joint) for a Crosslinked polyethylene-insulated Vinyl-sheathed power cable (CV cable), a measure for controlling or relieving the electric field stress at a cut and peeled end portion of an external semiconductor layer is needed. As such a measure, it has been practiced to mold a dielectric material such as a rubber composition having a higher permittivity than that of the cable insulator as an electric field relaxation layer between a conducting portion and an insulating portion at a terminal connection part or an intermediate connection part. This is a technology based on the utilization of a principle in which an equipotential line or electric stress is refracted when passing through substances which differ in permittivity.
As a dielectric material for use to form the electric field relaxation layer, there is known a dielectric material obtained by adding a carbon material such as carbon black to a polymeric material. This type of dielectric material has been unsuited to power cables, since the addition of the carbon material increases relative permittivity but would simultaneously impart conductivity. As a countermeasure against this problem, addition of silica-treated (silica-coated) carbon black to a silicone rubber composition has been proposed (Japanese Patent No. 4917164: Patent Document 1). Also, graft-bonding a vinyl polymer to carbon nanotubes has been proposed (Japanese Patent No. 5674143: Patent Document 2). In either of these techniques, however, it is necessary to form an insulating layer on surfaces of the carbon material before addition of the carbon material to the resin or rubber. Thus, these techniques are rather expensive in terms of materials cost and process cost.
As an approach other than the use of carbon materials, addition of a composite oxide of conductive zinc oxide and aluminum oxide to a silicone rubber composition has been proposed (Japanese Patent No. 4152672: Patent Document 3). In this case, however, it is necessary to add the composite oxide in a large amount, which leads to a raised specific gravity and a lowered strength. Further, composite insulator materials obtained by blending a high-permittivity substance such as barium titanate into a silicone rubber or an ethylene-propylene-diene copolymer have been proposed (JP-A 2001-266680: Patent Document 4). In this case, also, the problem of a raised specific gravity and a lowered strength is generated like in the above-mentioned cases.
Note that in related art of the present invention includes the following documents together with the above-mentioned.